Question

Draw Lewis structures for the following: (a) Phosgene, \(\mathrm{COCl}_{2}\), a poisonous gas (b) Hypochlorite ion, \(\mathrm{ClO}^{-},\) present in many swimming pool chemicals (c) Hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}\) (d) Sulfur dichloride, \(\mathrm{SCl}_{2}\)

Short answer

Each molecule is constructed with central atoms and surrounding atoms satisfy their octets using lone pairs.

Step-by-step solution

Identifying Valence Electrons

Count the total number of valence electrons for each compound or ion: (a) Phosgene, \( \text{COCl}_2 \): Carbon has 4, oxygen has 6, and each chlorine has 7 valence electrons.\[\text{Total} = 4 + 6 + 2(7) = 24 \, ext{electrons}\](b) Hypochlorite ion, \( \text{ClO}^- \): Chlorine has 7, oxygen has 6, and an extra electron for the negative charge.\[\text{Total} = 7 + 6 + 1 = 14 \, ext{electrons}\](c) Hydrogen peroxide, \( \text{H}_2\text{O}_2 \): Each hydrogen has 1, and each oxygen has 6 valence electrons.\[\text{Total} = 2(1) + 2(6) = 14 \, ext{electrons}\](d) Sulfur dichloride, \( \text{SCl}_2 \): Sulfur has 6, and each chlorine has 7 valence electrons.\[\text{Total} = 6 + 2(7) = 20 \, ext{electrons}\]

Drawing Skeletal Structures

(a) \( \text{COCl}_2 \): The carbon atom is central with oxygen double-bonded and chlorines single-bonded.(b) \( \text{ClO}^- \): Oxygen is single-bonded to chlorine.(c) \( \text{H}_2\text{O}_2 \): Oxygen atoms are single-bonded to each other, each with one hydrogen bonded to it.(d) \( \text{SCl}_2 \): Sulfur is central with two single bonds connecting to chlorine atoms.

Distributing Remaining Electrons

Distribute remaining electrons as lone pairs to complete octets:(a) \( \text{COCl}_2 \): 8 electrons for a double bond between carbon and oxygen, 6 electrons around each chlorine, and 4 electrons remain around oxygen to complete the octet.(b) \( \text{ClO}^- \): Complete the octets by placing 6 electrons around each atom; Oxygen will carry the negative charge.(c) \( \text{H}_2\text{O}_2 \): Each oxygen should have 2 lone pairs, satisfying their octets.(d) \( \text{SCl}_2 \): Each chlorine completes its octet with 6 lone electrons, and sulfur will have 2 lone pairs.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom. They play a key role in chemical bonding since these electrons can be shared or transferred between atoms. To understand how compounds are formed, we first need to count the valence electrons for each atom in the compound.
For example, in phosgene (COCl_2), we have:

• Carbon contributing 4 valence electrons
• Oxygen contributing 6 valence electrons
• Each chlorine contributing 7 valence electrons

This means we have a total of 24 valence electrons (4 + 6 + 2 \times 7 = 24) available for bonding and fulfilling the octet rule, which we’ll discuss later.
Similarly, for sulfur dichloride (SCl_2), sulfur contributes 6 valence electrons, and each chlorine contributes 7, which totals 20 (\(6 + 2 \times 7 = 20\)) electrons.
Understanding the total number of valence electrons is crucial for drawing the correct Lewis structures and for predicting how atoms will connect and share electrons.

Chemical Bonding

Chemical bonding involves the sharing or transfer of valence electrons between atoms to achieve stability. In Lewis structures, we often show these bonds as lines connecting the atoms.
Consider the structure of phosgene ( COCl_2 ):

• The carbon atom is central, usually forming multiple bonds due to its four valence electrons
• A double bond is formed between carbon and oxygen to fulfill both atoms’ electron requirements
• Single bonds form between carbon and each chlorine atom

The shared electrons in these bonds ensure that atoms are more stable, closely resembling the electron configuration of noble gases.
In compounds like sulfur dichloride, sulfur will commonly form single bonds with each chlorine, again striving to complete the valence shell and increase stability. The formation and type of bonds are determined by the number of electrons available and the requirement of each atom to achieve a full valence shell.

Octet Rule

The octet rule is a fundamental concept in chemistry, dictating that atoms tend to bond in such a way that they have eight electrons in their valence shell. This is similar to the electron configuration of a noble gas, which is naturally stable.
When we apply the octet rule to chemical compounds:

• Each atom seeks to fulfill this rule during bond formation
• For example, in hypochlorite ion ( ClO^- ), oxygen shares electrons with chlorine to complete its octet
• The extra electron from the negative charge also helps in achieving the necessary octet for the structure
• In hydrogen peroxide ( H_2O_2 ), even though hydrogen is an exception (requiring only two electrons for a full shell), each oxygen atom strives to maintain eight electrons

Understanding the octet rule allows us to predict the structure and stability of molecules. However, there are exceptions to this rule, such as when dealing with molecules containing hydrogen, boron, or larger atoms like phosphorus and sulfur, which can have expanded valence shells.